The specific interactions between blastocyst stage mouse embryos and mouse uterine epithelial cells were examined in hanging drop cultures with the light and electron microscope. Adhesion of blastocysts to dishes under the influence of various charged polymers was examined by measuring the morphology and degree of spreading of the tropoblast cells, as compared with cells isolated from tissues and with cell lines. The influence of adhesions (to epithelial cells or plastic) on the biosynthesis of oligosaccharides was examined by the incorporation of labeled glucosamine and gel exclusion chromatography. Our observations indicate that development of blastocyst stage mouse embryos, at least during the 5-day period examined, appears more like normal in vivo development than does development on a plastic substrate, where the trophoblast cells tend to spread. The adhesive response of the trophoblast cells on plastic dishes created with various poly-anions and poly-cations is not significantly different from that of other cell types despite the fact that in vivo the adhesiveness of the blastocyst to the uterine epithelium is hormonally regulated. The incorporation of glucosamine into oligosaccharides appears to be influenced by adhesion of the blastocyst to either plastic or epithelial cells. Studies in progress suggest that in both cases there is a pattern of elution from a gel column that is different from that seen with blastocysts which are prevented from attachment.